Do I need to reserve disk space for UFS snapshots?

Eric Anderson anderson at
Wed Apr 26 20:23:57 UTC 2006

Kent Ho wrote:
> Hi,
> I'm planing to use FreeBSD snapshots.  It seems to be the snapshop image 
> is placed on the same file system.  The question is how do I manage or 
> reserve space for the snapshots?  Can someone give me some guidelines or 
> pointers.
> Here's my netapp it has a separate area allocated for the snapshots, I'm 
> keeping 2 weekly snapshots in there(weekly.0 & weekly.1) using around 30G.
> Filesystem          kbytes       used      avail capacity  Mounted on
> /vol/vol0/          305052552  272495320   32557232    89% /vol/vol0/
> /vol/vol0/.snapshot  76263136   30658028   45605108    40% 
> /vol/vol0/.snapshot
> Filesystem               iused      ifree  %iused  Mounted on
> /vol/vol0/            10802387   89822586    11%   /vol/vol0/
> If copy all content in vol0 to a FreeBSD and make 2 snapshot will it be 
> around the same? 272G + 30G = 302G?  So if I df on the FreeBSD box 
> around 302G will be used?
> Any help and information is highly appreciated.

FreeBSD snapshots do not work the same way as NetApp snapshots.  First, 
they must reside on the same filesystem as the snapshotted filesystem. 
They will initially eat up some space upon creation equal to the space 
used by the cylinder group summaries and superblocks, but it isn't a 
whole lot.  You'll need to have however much space is changed weekly for 
each snapshot, so if 10Gb per week is changed, then you'll need that 
much space per snapshot file done.


Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.

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